Toroidal variable reactance transformer having two saturable cores



United States Patent 3,343,074 TOROIDAL VARIABLE REACTANCE TRANS- FORMER HAVING TWO SATURABLE CORES Elwood M. Brock, Flemington, NJ., assignor to Hunterdon Transformer Co., Flemington, N.J., a corporation of New Jersey Filed July 7, 1964, Ser. No. 380,770 8 Claims. (Cl. 323-56) ABSTRACT OF DISCLOSURE A variable transformer having three annular concentrically arranged cores and toroidally-wound control, primary and secondary windings. Two of the cores are saturable, and carry the DC control windings, while the main core carries the secondary winding. The unitary primary winding surrounds all three cores, and is wound on top of the DC and secondary windings.

Field 0 the invention The present invention relates to controlling devices designed to respond to direct current signals or to low frequency alternating current signals, and more particularly to those constructed with magnetic circuits.

Description of the prior art Many advances have been made in recent years in the field of magnetic controllers; however, many difficulties remain to be solved. The present invention, which comprises a high power transformer device having a reactance that is controllable by means of a variable direct current, is designed to overcome the disabilities of the prior art.

Prior transformer devices have utilized the saturating effects of direct current to derive a controllable output voltage and current from transformer devices. Such prior attempts, however, have not solved the control problems satisfactorily, for they retain a relatively high reactance level even when the saturating current is at a maximum value and the transformer is in its saturated condition. Such prior devices are also unsatisfactory inasmuch as they do not attain a high degree of efficiency. This is primarily due to the large amount of flux leakage which occurs between those portions of the transformer core on which the primary and secondary windings are wound and those portions of the core which are used for control purposes.

Another problem that often occurs in controllable transformers is that of leakage of an alternating current into the direct current control circuit. Such leakage may reach such high values as to be destructive of the direct current source, in addition to reducing the efficiency of the transformer, and should therefore be avoided.

Summary of the invention It is an object of the present invention to overcome the disadvantages of the prior art and to provide a variable transformer which will both control power and transform voltage and current in a single, unitary structure.

A further object of the invention is to provide a variable reactance transformer having improved response time, power factor and efliciency as compared to prior art devices.

Another object of the invention is to provide a transformer device having a controllable reactance which, when manufactured, effects substantial savings in both material and labor to produce a considerable reduction in cost while providing an improved device of higher efiiciency.

A further object of the invention is to provide a variable reactance device in which fiux leakage is reduced substantially to zero.

Another object of the invention is to provide a transformer device having a controllable output in which alternating current signals and transients are prevented from reaching the source of DC control current.

An additional object of the invention is to provide a variable reactance transformer which,- through the use of annular core elements, provides efficient coupling and a very low reactance in its fully saturated condition and an extremely high reactance in its unsaturated condition.

Another object of the invention is to provide a controllable reactance transformer which includes three annular core members axially aligned and located adjacent one another, the three cores comprising a main transformer core and two auxiliary cores, one auxiliary core being located on each side of the main core. A primary winding for the transformer is coupled with all three cores while a secondary winding is coupled only to the main core. A direct current control winding is coupled to each of the auxiliary cores, the .control winding being connected in series with a variable source of control power. Variation of the DC control current drives the auxiliary cores between a fully saturated condition and an unsaturated condition, the cores presenting a low reactance to current flow in the primary winding of the transformer when fully saturated and presenting a high reactance to the flow of primary current when unsaturated. Gradual variation of the direct current so that the auxiliary cores go from a fully saturated condition to an unsaturated condition will vary the output of the secondary winding from a high level to a low level. Through the use of annular cores surrounded by toroidal windings, leakage flux is almost entirely eliminated, resulting in a high efliciency device.

Brief description of the drawings The novel features which are a characteristic of the invention are set forth with particularity in the appended claims, but the invention and its objects will be understood more clearly and fully from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic illustration of a transformer device constructed according to the present invention;

FIG. 2 is a diagrammatic showing of the physical relationship of the several cores and coils;

FIG. 3 is a transverse section of a transformer according to the present invention; and

FIG. 4 is a transverse section of a second embodiment of a variable reactance transformer in accordance with the present invention.

Description of the preferred embodiments Turning now to the schematic diagram of FIG. 1, the numeral 10 generally indicates a variable reactance transformer circuit connected in accordance with the principles of the present invention. The transformer is comprised of a main core 12 and two auxiliary cores 14 and 16. These cores are annular in shape and are composed of any suitable magnetic material. They are so proportioned that the auxiliary cores 14 and 16 may become saturated when under the influence of a normal range of DC control current. Neither the auxiliary cores nor the main core necessarily exhibit the so-called squareloop hysteresis curves, but preferably are of conventional magnetic materials.

Auxiliary cores 14 and 16 carry control windings 18 and 20, respectively, connected in series opposition across a variable source of DC control current connected at terminals 22 and 24. The series arrangement of control windings is shunted by an impedance 38.

Surrounding all three of cores 12, 14 and 16 is the unitary primary winding 26 which may be connected at terminals 28 and 30 to a source of alternating current. As shown, windings 26 are not separately wound on each of the three cores, but each turn of the primary winding passes around all three cores. This primary winding normally carries the input current to the device, the current which is to be transformed and/ or controlled.

A secondary Winding 32 is wound on the main core 12 only and carries the controlled and/ or transformed current to a load device connected between terminals 34 and 36.

FIG. 2 illustrates more clearly the relationship between the several cores and between the cores and their respective windings. The elements of FIG. 2 which correspond to those of FIG. 1 carry corresponding numerals. As may be seen, the main core 12 is sandwiched between the auxiliary cores 14 and 16, the cores being as closely adjacent one another as the bulk of the windings will permit. The cores are axially aligned in this embodiment. The control windings 18 and 20 are wound on the auxiliary cores only, entirely by-passing main core 12. Primary winding 26 is inductively coupled to all three cores, each turn of the winding passing around all three cores. Secondary winding 32 is wound only on the main core 12 and thus is isolated from the direct current control flux induced in the auxiliary cores by the DC control source.

FIG. 3 is a more complete showing of the device of FIGS. 1 and 2, disclosing the manner in which the toroidal windings are wound around the several core elements. Thus, FIG. 3 discloses a main core element 40 which is annular in shape and surrounded by a toroidal secondary winding 42. Leads 44 and 46 are connected to the secondary winding and carry the induced output current to a load device. Auxiliary cores 48 and 50 are axially aligned with the main core and are located on opposite sides thereof. These cores carry control windings 52 and 54, respectively. Core elements 48 and 50 are annular in shape and of smaller cross sectional area than the main core 40. Control windings 52 and 54 are wound around their respective core elements to form toroids and are serially connected by means of conductor 56. Terminal leads 58 and 60 provide input connections to the control windings 52 and 54. Surrounding all three core elements and their respective windings is the primary winding 62 which also is toroidal in form. Input leads 64 and 66 are connected to the primary winding and carry the alternating 'current input signal which is to be transformed and/ or controlled.

The operation of the variable reactance transformer will now be described with reference to the illustrations of FIG. 1 and FIG. 2. The source of alternating current that is to be transformed and controlled is connected at terminals 28 and 30 to the primary winding 26. Considering first the situation where no control voltage is applied across terminals 22 and 24, the primary current flowing in winding 26 induces a reversing flux in each of cores 12, 14 and 16. By reason of the unsaturated condition of the core elements 14 and 16, and by reason of the crosssectional area of the auxiliary cores, an extremely high reactance is presented, resulting in a very low flow of primary current, a small amount of flux change in the main core 12, an negligible amount of induced current in the secondary winding 32. A small amount of alternating current is also induced in each of the control cores 18 and 20; however, by reason of the fact that these coils are connected in series opposition, these induced currents tend to cancel themselves out. Any unbalance between the voltages induced in coils 18 and 20 will result in a small current flow through impedance 38 which shunts the DC control source and provides a path for the induced alternating currents.

If, now, a direct current control source of sufficient power to drive auxiliary cores 14 and 16 into a saturated condition is applied across terminals 22 and 24, a relative- 1y low reactance will be presented to the source of primary current. Since none of the primary current will now be needed to drive the auxiliary cores 14 and 16 toward saturation, virtually all of this current will be used to establish alternating flux in the main core 12, thus inducing a maximum amount of alternating current in the secondary winding 32. Under these conditions, the device acts as a high efliciency transformer, providing voltage and current at terminals 34 and 36 which are proportional to the source of primary current in accordance with the ratio of coils 26 and 32. If any alternating current is induced in either of coils 18 and 20, these voltages will again tend to be cancelled out, with any dilference being dissipated in impedance 38. 7

By making the DC control source variable, any desired degree of saturation may be attained in cores 14 and 16. Thus, varying amounts of the energy available from the primary alternating current source are absorbed by core elements 14 and 16 in proportion to the degree of saturation, leaving varying amounts of energy to drive the main core 12. The reactance of the device, therefore, is varied by means of the DC control source, and the energy available for driving the secondary winding 32 is also so varied. The output of the secondary winding 32 may be controlled easily and quickly merely by varying the DC control source which, in turn, varies the reactance of the primary winding of the transformer.

By utilizing two auxiliary cores, several advantages are obtained. For example, the secondary winding 32 is isolated from the DC control flux. This produces a higher efficiency transformer inasmuch as the flux in the magnetic circuit of the secondary winding does not leak away to be dissipated in the control windings. Further, the use of two auxiliary cores and series opposed control windings permits easy balancing out of alternating current voltages in the control circuit, reducing the possibility of damaging the DC control source. The use of three annular cores is also of advantage in high power applications such as are contemplated for the present device inasmuch as the auxiliary cores can be designed for lower power requirements than is required for the main core element and only the main core need be designed to carry full power. The use of annular core elements substantially reduces the leakage flux which is often an appreciable factor in cores of other configurations.

The impedance 38 shunting the control windings 18 and 20 is an important feature of the present device, for although the coils 18 and 20 are reverse-connected so as normally to cancel out the alternating current flux induced by the primary current in the auxiliary cores, there nevertheless often occurs a high resultant voltage at the input terminals of the control windings. This high resultant voltage may be caused by any one or a combination of several factors. For example, the auxiliary cores normally are not identical so that the alternating current from the primary winding will induce unequal fluxes. If the windings 18 and 20 have the same number of turns, the unequal fluxes induced in 14 and 16 will induce unequal voltages in the coils. If there is a substantial difference between the cores, the resultant voltage can become appreciable, and in a high power device may go as high as two thousand volts. A similar result occurs if the cores are equal but the windings difier either in number of turns, resistance of the wire, or the like.

Even when the windings and cores are substantially balanced, a resultant voltage can appear as a transient across the control coils when the device is first-turned on, the transient occurring during the first half cycle or so of the primary alternating current. This transient is caused by the primary source momentarily inducing flux in only one of the auxiliary cores. After a few cycles of the primary current, the voltages induced in the two auxiliary cores will tend to balance out; however, this transient can produce a substantial voltage.

One way of reducing the difference voltages appearing across the control windings is to wind the control auxiliary cores with different numbers of turns, the number of turns being proportional to the amount of magnetic material in the core. The cores are first matched as closely as possible, then the coils are wound proportionally to obtain a similar number of ampere-turns for each of the two cores. A rough way to obtain a close result is to proportion the coils by weight, the coils being wound with a variable number of turns until the total weight of one core with its associated coil is equal to the total weight of the other core-coil assembly.

shunt impedance for the control circuit is shown in this figure as being a capacitor 90. However, as has been noted, this impedance may also be a low value resistance.

In its normal configuration, the transformer of the present invention normally would be potted, or encapsulated in a suitable material and would be provided with 'means for water cooling.

10 KW. VARIABLE REACTANCE TRANSFORMER [Load resistance.08 ohm at 8.5 kw.]

The unbalanced voltage can be further reduced by connecting an impedance across the terminals of the DC control winding. The best impedance for this purpose is a capacitor of large capacity which will provide a short circuit for alternating current voltages generated in the control windings and will prevent the AC from affecting where V. is volts, A. is amperes, KVA. is kilovolt-amps, and KW. is kilowatts.

The foregoing table is taken from a device operated Without water cooling. When self cooling is used, the unit must be operated at a lower level. When water cooling is used, the'following values are available:

10 KW. VARIABLE REACTANCE TRANSFORMER [Load resistance-.18 ohm at 12 kw.]

Line Load Control Power Factor, percent V. A. KVA. V. A. KW. V. A. Watts the DC control source. However, the use of a capacitor tends to reduce the overall life of the variable reactance transformer, for although the magnetic circuits will operate maintenance-free for a long period of time, electrolytic capacitors generally do not have a long life. The necessity of replacing capacitors would make the device less reliable and reduce its usefulness for application in remote and hard to reach areas. A less efficient but more stable method of reducing the unbalance voltage is the replacement of the capacitor by a low resistance. This is not as satisfactory as the capacitor since the resistance acts as a load for the DC control source while a capacitor has no such elfect, but a resistor normally has a very long life.

Referring now to FIG. 4 of the drawings, there is shown a second embodiment of the variable reactance transformer according to the present invention. In this embodiment, the main core element 70, surrounded by the secondary winding 72, is located within the diameter of annular auxiliary cores 78 and 80 and their respective control windings 82 and 84. The main core 70 is annular and is concentric with the axes of the auxiliary cores, i.e., is coaxial with them, as in the embodiments described hereinabove, but is so arranged as to provide a different configuration. Output leads 74 and 76 are connected to 70 secondary coils 72 while leads 86 and 88 carry the variable DC control current to the auxiliary windings 82 and 84. Toroidal primary winding 92 surrounds the three cores with their individual windings. Input leads 94 There has thus been described a variable reactance transformer having improved response time, power factor and efficiency and which, by reason of the use of toroidal cores which simplify the manufacturing process and which reduce losses within the core, effects considerable savings in both material and labor. It is obvious that this transformer can be adapted for operation with three-phase current and that the particular physical relationship of the various cores and coils could be changed without departing from the spirit of the invention. For example, the main core of FIG. 4 could be placed outside the auxiliary cores 78 and 80, or the windings could be arranged differently on the cores by making them less than fully toroidal. It will be apparent that either the primary or the secondary windings could be multiple-voltage windings; i.e., could be made up of two or more separate windings connectable in series or parallel arrangements. With such an arrangement, the variable reactance transformer can be adapted for use with various input voltages and can produce output voltages in varying ranges. It should be noted that in the foregoing tables showing values taken from working models of the device, a dual secondary Winding transformer was used. This dual secondary was parallel connected for the values obtained in the first table and series connected for the second table. Other variations and modifications will be apparent to those skilled in the art, and it is therefore desired that the foregoing description be taken as illustrative and limited and 96 connect a source of primary current thereto. The only by the following claims.

I claim:

1. A variable reactance transformer comprising a main core, a pair of auxiliary cores located adjacent said main core, a secondary winding wound on said main core only, a control winding wound on each of said auxiliary cores, said control windings being connected in series opposition, a primary winding surrounding said main and auxiliary cores, each turn of said primary winding passing around all of said cores, and a variable source of direct current connected across said series-connected control windings whereby the saturation of said auxiliary cores may be controlled.

2. The variable reactance transformer of claim 1, wherein said auxiliary cores are axially aligned with 'said main core and located on opposite sides thereof.

3. The variable reactance transformer of claim 1, wherein said main core and said auxiliary cores are annular.

4. The variable reactance transformer of claim 3, wherein said control windings form toroids on said auxiliary cores, said secondary winding forms a toroid on said main core, and said primary winding forms a toroid surrounding said auxiliary and main cores and surrounding said control and secondary windings.

5. The variable reactance transformer of claim 4, wherein said auxiliary cores are axially aligned with said main core and located on opposite sides thereof.

6. The variable reactance transformer of claim 4, wherein said auxiliary cores are coaxial with said main core and located outside said main core.

7. In a high-power variable reactance transformer, an annular main core having an axis and a pair of annular auxiliary cores coaxial with said main core and adjacent thereto, a control winding surrounding each of said auxiliary cores and connected in series opposition, a secondary winding surrounding only said main core for isolation from said control windings, and a unitary primary winding surrounding all of said cores, a variable source of direct current connected across the series arrangement of said control windings to drive the auxiliary cores between a saturated and an unsaturated condition, said transformer having a low reactance in said saturated condition and a high reactance in said unsaturated condition, and an impedance connected across said series arrangement of control windings.

8. The transformer of claim 7, wherein each turn of said primary winding passes around all said cores, each winding of said transformer forming a toroid around its respective core or cores.

References Cited UNITED STATES PATENTS 2,586,657 2/1952 Holt 323-56 X 2,870,397 1/1959 Kelley 323-56 3,123,764- 3/ 1964 Patton 323-56 JOHN F. COUCH, Primary Examiner.

A. D. PELLINEN, Assistant Examiner. 

7. IN A HIGH-POWER VARIABLE REACTANCE TRANSFORMER, AN ANNULAR MAIN CORE HAVING AN AXIS AND A PAIR OF ANNULAR AUXILIARY CORES COAXIAL WITH SAID MAIN CORE AND ADJACENT THERETO, A CONTROL WINDING SURROUNDING EACH OF SAID AUXILIARY CORES AND CONNECTED IN SERIES OPPOSITION, A SECONDARY WINDING SURROUNDING ONLY SAID MAIN CORE FOR ISOLATION FROM SAID CONTROL WINDINGS, AND A UNITARY PRIMARY WINDING SURROUNDING ALL OF SAID CORES, A VARIABLE SOURCE OF DIRECT CURRENT CONNECTED ACROSS THE SERIES ARRANGEMENT OF SAID CONTROL WINDINGS TO DRIVE THE AUXILIARY CORES BETWEEN A SATURATED AND AN UNSATURATED CONDITION, SAID TRANSFORMER HAVING A LOW REACTANCE IN SAID SATURATED CONDITION AND A HIGH REACTANCE IN SAID UNSATURATED CONDITION, AND AN 